Observations of a faint planet-sized star help weigh in on its millisecond pulsar companion.
A dense, collapsed star has shattered and consumed almost the entire mass of its companion star and, in the process, grown into the heaviest neutron star yet observed. It rotates at 707 times per second — making it one of the fastest rotating neutron stars in the Milky Way galaxy. Weighing in on this record-setting neutron star, which tops the charts at 2.35 solar masses (the mass of our sun), helps astronomers understand the strange quantum state of matter inside these extremely dense objects. If they get much heavier than this, neutron stars collapse completely and disappear as a black hole. “We know approximately how matter behaves at nuclear densities, such as in the nucleus of a uranium atom,” said Alex Filippenko, Distinguished Professor of Astronomy at the University of California, Berkeley. “A neutron star is like a giant nucleus, but when you have a solar mass and a half of this material, which is about 500,000 Earth masses of nuclei all together, it’s not at all clear how they’re going to behave.” According to Roger W. Romani, Professor of astrophysics at Stanford University, neutron stars are incredibly dense, with 1 cubic inch weighing more than 10 billion tons. This means that their cores are the densest matter in the universe without black holes, which are impossible to study because they are hidden behind their event horizons. As such, the neutron star, a pulsar called PSR J0952-0607, is the densest object visible from Earth. Astronomers have measured the speed of a faint star (green circle) that has been stripped of nearly all of its mass by an unseen companion, a millisecond neutron star and pulsar that they determined is the most massive yet found and perhaps the upper limit for neutron stars. The objects are located in the constellation Sextans. Contributors: WM Keck Observatory, Roger W. Romani, Alex Filippenko It was the extraordinary sensitivity of the 10-meter Keck I telescope at Maunakea in Hawaii that made it possible to measure the mass of the neutron star. It captured a spectrum of visible light from the hotly glowing companion star, which has now shrunk to the size of a large gas planet. Located in the direction of the constellation Sextans, the stars are about 3,000 light-years from Earth. Discovered in 2017, PSR J0952-0607 is referred to as a “black widow” pulsar. Their name is an analogy to the tendency of female black widow spiders to consume the much smaller male after mating. Hoping to determine the upper limit on how large neutron stars/pulsars can grow, Filippenko and Romani have been studying black widow systems for more than a decade. “By combining this measurement with those of several other black widows, we show that neutron stars must reach at least this mass, 2.35 plus or minus 0.17 solar masses,” said Romani, a professor of physics in the School of Humanities and Sciences of Stanford. and member of the Kavli Institute for Particle Astrophysics and Cosmology. “In turn, this provides some of the strongest constraints on the properties of matter at many times the density seen in atomic nuclei. Indeed, many different popular models of physical dense matter are ruled out by this result.” If 2.35 solar masses is near the upper limit of neutron stars, astronomers say, then the interior is likely to be a soup of neutrons as well as up and down quarks—the components of normal protons and neutrons—but not exotic matter. such as “strange” quarks or kaons, which are particles containing a strange quark. “A high maximum mass for neutron stars suggests that they are a mixture of nuclei and their dissolved up and down quarks down to the core,” Romani said. “This rules out many proposed states of matter, especially those with exotic internal compositions.” Romani, Filippenko and Stanford graduate student Dinesh Kandel are co-authors of a paper describing the team’s results published today (July 26, 2022) in The Astrophysical Journal Letters.
How big can they grow?
Astrophysicists generally agree that when a star with a core larger than about 1.4 solar masses collapses at the end of its life, it forms a dense, compact object with an interior under such high pressure that all the atoms collapse to form a sea of neutrons. and their subnuclear components, the quarks. These neutron stars are born spinning, and although they are too faint to be seen in visible light, they reveal themselves as pulsars, emitting beams of light—radio waves, X-rays, or even gamma rays—flashing Earth as they spin, like the spinning beam of a lighthouse. “Normal” pulsars spin and flash about once a second, on average, a rate that can be easily explained given the normal rotation of a star before it collapses. But some pulsars repeat hundreds or even 1,000 times a second, which is hard to explain unless matter has crashed into the neutron star and is spinning it. But for a few millisecond pulsars, no companion is visible. One possible explanation for the individual millisecond pulsars is that each once had a companion, but stripped it down to nothing. “The evolutionary path is absolutely fascinating. Double exclamation point,” Filippenko said. “As the companion star evolves and begins to become a red giant, material is poured into the neutron star and this spins up the neutron star. Spinning upwards, it now gains incredible energy and a wind of particles begins to emerge from the neutron star. This wind then hits the donor star and begins to remove material, and over time, the mass of the donor star decreases to that of a planet, and if even more time passes, it disappears altogether. So, this is how millisecond pulsars could be created. They weren’t alone at first – they had to be in a dyadic pair – but gradually they evaporated their partners and now they’re lonely.” The pulsar PSR J0952-0607 and its faint companion star support this origin story for millisecond pulsars. “These planet-like objects are the remnants of normal stars that have contributed mass and angular momentum, spinning their pulsar companions over millisecond periods and increasing their mass in the process,” Romani said. “In the case of cosmic ingratitude, the black widow pulsar, having swallowed a large part of its companion, is now heating and vaporizing the companion into planetary masses and perhaps complete annihilation,” Filippenko said.
Spider pulsars include redbacks and tindarens
Finding black widow pulsars in which the companion is small, but not too small to detect, is one of the few ways to weigh neutron stars. In the case of this binary system, the companion star—now only 20 times the mass of Jupiter—is warped by the mass of the neutron star and tidally locked, similar to the way our moon is locked in orbit, so that we see only one side. The side facing the neutron star is heated to temperatures of about 6,200 Kelvin, or 10,700 degrees Fahrenheit, slightly hotter than our sun and bright enough to see with a large telescope. Filippenko and Romani turned the Keck I telescope on PSR J0952-0607 on six occasions over the past four years, each time observing with the Low-Resolution Imaging Spectrometer in 15-minute chunks to catch the faint companion at specific points in 6’s orbit. 4 hours of the pulsar. By comparing the spectra with those of similar sun-like stars, they were able to measure the orbital velocity of the companion star and calculate the mass of the neutron star. Filippenko and Romani have looked at about a dozen black widow systems so far, though only six had companion stars bright enough to allow them to calculate a mass. All the neutron stars involved are less massive than the pulsar PSR J0952-060. They hope to study more black widow pulsars, as well as their cousins: the redbacks, named after the Australian equivalent of black widow pulsars, which have companions closer to one-tenth the mass of the sun. and what the Romans called tidarrens – where the partner is about one hundredth of the solar mass – from a relative of the black widow spider. The male of this species, Tidarren sisyphoides, is about 1% the size of the female. “We can continue to look for black widows and similar neutron stars skating even closer to the black hole’s rim. But if we don’t find any, it strengthens the argument that 2.3 solar masses is the real limit, beyond which black holes form,” Filippenko said. “This is right at the limit of what the Keck telescope can do, so barring fantastic observing conditions, tightening up the measurement of PSR J0952-0607 will likely await the era of 30-meter telescopes,” Romani added. Citation: “PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star” by Roger W. Romani, D. Kandel, Alexei V. Filippenko, Thomas G. Brink and WeiKang Zheng, 26 July 2022, The Astrophysical Journal Letters.DOI : 10.3847/2041-8213/ac8007 Other co-authors of the ApJ Letters paper are UC Berkeley researchers Thomas Brink and WeiKang Zheng. The work was supported by the National Aeronautics and Space Administration (80NSSC17K0024, 80NSSC17K0502), the Christopher R. Redlich Fund, the TABASGO Foundation, and the UC Berkeley Miller Institute for Basic Science Research.
